1. Field of the Invention
The present invention relates generally to toothed belts and, more specifically, to a relatively thick jacket, thin cord, toothed belt.
2. Description of the Related Art
It is known to provide a toothed belt for an engine such as a synchronous belt that transmits load and motion by the action of molded teeth in grooves in a pulley for the engine. Typically, the toothed belt includes a belt body, a jacket disposed along at least one toothed peripheral surface of the belt body, and a tensile member embedded in the belt body. The belt body has a plurality of belt teeth formed of the body and spaced apart at a predetermined pitch. The tensile member is comprised of one or more cords embedded in the belt body.
It is also known that the toothed belt and pulley have certain geometrical relationships. For example, as illustrated in FIG. 1, a pulley 6 has an outside diameter (OD) and a pitch diameter (PD) describing a circular pitch (CP) or tooth repeat distance, relative to the center (C) of the pulley 6. A belt 8 has a belt pitch line (BPL) or neutral axis also describing a CP. The CP of the belt must match the CP of the pulley for proper engagement or fit. The pulley 6 has a pitch line differential (PLD) that is half the difference between the PD of the pulley 6 and the OD of the pulley 6. The belt 8 has a PLD that is equal to the distance between the BPL and a belt land. The land is the bottom surface of the groove between adjacent longitudinally spaced teeth of the belt and is indicated by reference numeral 20 in FIG. 3. Typically, a center or neutral axis of the tensile member lies, or is assumed to lie, along the BPL of the belt.
It is further known that engine designers maintain standard PLD and pitch line differentials in the design of the engine. Exemplary standards include SAE J1278 and ISO 9010 for automotive under-hood synchronous drives and RMA IP-24 and ISO 5296-1 for industrial synchronous drives. The PLD and pitch are critical dimensions that insure a sufficient engagement between the teeth of the belt and the cooperating pulley. As a result, belts designed for existing engines conventionally have to incorporate a standard PLD. It is general practice to design the PLD of the belt to match that of the pulley system, so that BPL and PD coincide as shown in FIG. 1, within practical tolerances. The pitch of the belt is generally controlled by properly selecting the mold dimensions. The PLD of the belt is generally controlled by the dimensions and properties of the tensile member and jacket. Nevertheless, some engine designers have designed belt drive systems incorporating various pulleys that do not match each other and/or that have non-standard PLD, thus placing severe stresses on the belt.
Moreover, automotive designers are demanding that overhead cam belt systems also drive additional components such as water pumps, fuel pumps, and the like placing increasingly severe loads on the belt. Previously, to boost the load carrying capability of a synchronous belt, a physical property of the elastomeric compound for the belt body was changed, generally reducing belt flexibility. Alternately, or in addition, changes in jacket and/or tensile member construction or use of newer, high-performance fiber materials have been investigated. However, a change in the dimensions of the tensile member or jacket to allow more flexibility and/or load capacity in the belt will result in a change in the PLD, with the potential of adversely affecting pitch fit. Thus, although changes to the tensile member or jacket could also favorably affect load capacity, the aforementioned geometrical restrictions have placed rather tight limitations on the practical extent of such changes
The interaction between the belt body, the tensile member, and the jacket is therefore important for the belt geometry, operation in a given drive system, and life and performance of the belt. Examples of such interaction and prior attempts to manipulate cord and jacket variables to achieve performance improvement have been disclosed. In U.S. Pat. No. 4,721,496 to Yokoyama et al., a very narrow range of fiberglass cord diameter, 0.9 to 1.1 millimeters (mm), combined with a cord diameter to jacket thickness ratio in the range from 1.8 to 5.0, was disclosed. In U.S. Pat. No. 5,531,649 to Osako et al., belts with an above-nominal PLD of between 0.73 and 0.85 mm, combined with a jacket thickness of between 0.3 and 0.5 mm, with glass or aramid cord, were disclosed. In U.S. Pat. No. 5,209,961 to Yokoi, belts with glass cord diameters of 0.63 mm to 0.85 mm, combined with a cord diameter to jacket thickness ratio in the range from about 1.5 to 2.2, were disclosed. Thus, changes in one of these components may not be facilitated unless consideration is made for the other components. Yet, performance of belts designed within these conventional parameter ranges have failed to meet desired performance standards, and in some cases, have failed to achieve even the minimum expected potential of newer, high-performance materials such as carbon fiber. As such, it is desired to focus on changes of the tensile member and jacket, which complement one another to achieve a desired belt geometry and improvement in performance.
Therefore, it is desirable to provide a new belt construction for a synchronous belt that focuses on the tensile member and jacket to achieve a desired belt geometry and improvement in performance, including improved load capacity and flexibility. It is also desired to provide a new belt construction for a synchronous belt that uses smaller diameter cords for the tensile member. It is further desirable to provide a new belt design approach that results in improved performance from high-strength, high-modulus cord materials. It is further desirable to provide a new belt construction that functions properly in a variety of pulleys within a predetermined pitch line differential range. Therefore, there is a need in the art to provide a toothed belt that meets at least one of these desires.